Conventional night vision systems, e.g. night vision goggles, use the reflected light from an object or scene by intensifying the image reflection and generating a monochrome image. The generated image is represented by monochromatic shades of intensity. More specifically, these types of systems use the image intensifier as an optical amplifier. Emitted photons, due to the illuminated image, that strike the entrance to the intensifier are converted to electrons by a photo-cathode. An electric field applied between the photo-cathode and Micro Channel Plate (MCP) accelerates the electrons. The electric field is also applied through the MCP itself. The electrons are accelerated through the MCP, which amplifies the number of incoming accelerated electrons that ultimately bombard monochrome phosphors on a phosphorous screen. The phosphorous screen converts the amplified electrons back to photons, thereby displaying an amplified image represented by green shades of intensity. An image sensor, e.g. CCD (Charged Coupled Device) or CMOS (Complimentary Metal Oxide Semiconductor) Imager, detects the amplified image and translates it into a monochrome video signal.
Methods for producing a color night vision system have been attempted. The paper entitled, “Fusion of Multi-Sensor Imagery for Night Vision: Color Visualization, Target Learning and Search” by Fay et al., provides a methodology for creating a color night vision image by combining the imagery from multiple sensors. Each sensor is associated with one specific frequency band, e.g. visible, SWIR (Short Wave Infra Red), LWIR (Long Wave Infra Red), and MWIR (Medium Wave Infra Red), that contains unique image information. There are two processing stages, included in the combination of the frequency band image information, that lead to the formation of color components, e.g. R (Red), G (Green), B (Blue), Y (Brightness), I (red-green contrast), Q (blue-yellow contrast), which comprise the final color image. Firstly, the image revealed from each sensor is registered, filtered from noise, and contrast enhanced. The second stage consists of contrast enhancement between the different images, thereby separating the complimentary information that each band contains while combining common information from the spectral bands. The final color image may be displayed when taking into account the combined information from all spectral bands.
Reference is made to U.S. Pat. No. 5,162,647 issued on Nov. 10, 1992 to R. J. Field, Jr. and entitled, “Color Image Intensifier Device Utilizing Color Input and Output Filters Being Offset By a Slight Phase Lag”. The patent includes an image intensifier tube normally providing a monochrome output. There exist input and output color filters for filtering desired light frequencies. Using a time interval methodology, the input color filter filters out the desired color frequency from the incoming light to the tube. The monochrome tube output is filtered through the output color filter to produce the corresponding input color component. Another embodiment of the patent uses spatial color filters, for e.g. colored fiber optic cables or colored micro-lenses, in both the input and output of the image intensifier. This allows for desired colors to be filtered and displayed in adjacent pixels to produce a color image. In another embodiment of the patent, only an input color filter such as a filter wheel, a sequential filter, or micro-lenses is used. An imager, e.g. CCD, is coupled to the output of the image intensifier with each pixel corresponding to a distinct color. The adjacent pixels may be combined to form a color image.
Reference is further made to U.S. Pat. App. No. 20020175268 filed on May 22, 2001 to Arlynn Walter Smith and entitled, “Color Night Vision Apparatus”. Each desired color, e.g. the three primary colors, has a specific image intensifier tube associated with it. The desired frequency from the low light image is input to each tube via a frequency splitter, thereby causing each tube to provide an intensified output of the desired frequency. The intensified output signals, associated with each tube, are combined to form a color image that is in fact the intensified low light input signal to each of the image intensifiers.
Known methodologies for producing a color night vision system depend on filtering the color frequencies in the white visible light. However, white light is lost during transmission through a filter. Typically, the Near InfraRed (NIR) portion of the spectrum is filtered out. This causes a brightness reduction of the low light input signal. Moreover, in the color systems described above, for each monochrome pixel there exist several pixels associated with it, e.g. three pixels for each primary color, one for each desired color. For pixels that are the same size as the monochrome pixel, the resolution of the colored image is diminished. Additionally, many of the aforementioned methods include the use of white phosphors that do not maintain the same brightness as monochrome phosphors. Brightness reduction and diminished resolution lead to a mediocre night vision color image.
Furthermore, using multiple image intensifying components or sensors, as described above, increases the size of the color night vision system.
It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements.